1. Field of Application
This disclosure relates to a drive circuit configured as an integral unit, for driving a voltage-control type of semiconductor switching device.
2. Background Technology
As described for example in Japanese patent No. 3430878, types of drive circuit are known for use in for on/off switching of an IGBT (insulated gate bipolar transistor) is which functions as a switching device in an electrical power converter circuit, whereby the IGBT is set in the on state by supplying a charging current to the gate of the IGBT (i.e., to charge the gate capacitance) via a charging circuit path, and is switched to the off state by passing a discharge current from the gate via a discharge circuit path. The gate voltage is thereby raised to an on-state value by supplying charge to the gate, and is lowered to an off-state value by discharging the gate charge. Abrupt initiation or abrupt interruption of a flow of collector current of the IGBT is thereby avoided.
Stray capacitances exist between the gate and the emitter and collector of the IGBT, and any noise voltages between the emitter and collector may become applied to the gate via the stray capacitances. This may cause variations in the gate voltage while the IGBT is in the off state, causing a danger that the IGBT may accidentally enter the on state.
To overcome this problem it has been proposed to provide a switching device (referred to in the following as an off-holding switching device) connected in a circuit path other than the charging circuit path and the discharge circuit path, which can be controlled to apply a short-circuit between the gate and emitter of the IGBT, and an off-holding circuit for selectively setting the off-holding switching device in on and off states. That is, the off-holding switching device is set in the on (short-circuit) state while the IGBT is required to be in the off state, and set in the on (open-circuit) state while the IGBT is to be in the on state. Accidental switch-on of the IGBT can thereby be prevented.
However the assignees of the present invention have found a basic problem with that method of preventing accidental switch-on. As described hereinafter referring to a specific example, the off-holding switching device may accidentally enter the on state during discharging of the gate capacitance of the IGBT, before the gate capacitance has become completely discharged. Thus a short-circuit may be prematurely applied between gate and emitter of the IGBT, before the gate voltage has become sufficiently low IGBT. This can result in abrupt interruption of the collector current of the IGBT, causing voltage surges to be applied to the IGBT, and hence possibly causing damage or reduced reliability.